Method of producing high tensile strength aluminum coated ferrous strands



Sept. 1, 1970 M. B, PIERSON ET AL 3,526,529

METHOD OF PRODUCING HIGH TENSILE STRENGTH ALUMINUM COATED FERROUSSTRANDS Original Filed Sept. 18, 1964 INVENTOR. MARVIN B.Pu=.zsou moEmu: LKHAPP, BY wh m Z/W' arronuzvs,

United States Patent 3,526,529 METHOD OF PRODUCING HIGH TENSILE STRENGTHALUMINUM COATED FER- ROUS STRANDS Marvin B. Pierson, Middletown, Ohio,and Earle L. Knapp, Kansas City, Mo., assignors to Armco SteelCorporation, Middletown, Ohio, a corporation of Ohio Continuation ofapplication Ser. No. 397,538, Sept. 18, 1964. This application Apr. 9,1969, Ser. No. 815,529 Int. Cl. C23c N08 US. Cl. 11751 4 Claims ABSTRACTOF THE DISCLOSURE Process for producing high tensile strength aluminumcoated steel wire, wherein the surface of the wire is thoroughly cleanedand then brought to a bath of molten aluminum coating metal at atemperature equal to or below that of the bath so that the surface ofthe strand is heated to a temperature not substantially greater than themelting point of the coating metal during its immersion therein.

CROSS REFERENCE TO RELATED APPLICATIONS This is a continuation ofapplication Serial No. 397,538, filed Sept. 18, 1964 entitled Method ofProducing Metallic Coated Ferrous Strands, now abandoned, in the namesof Marvin B. Pierson and Earle L. Knapp.

BACKGROUND OF THE INVENTION The fundamental nature of this invention canperhaps best be understood by referring to a specific article which maybe produced by the instant process more economically than has heretoforebeen possible. For example, ACSR high tension cable (AluminumConductorsfiSteel Reinforced) consists of a plurality of aluminum wireswound about a central steel reinforcing wire or wires. This steelsupport wire must possess a minimum tensile strength of of 165,000 to185,000 p.s.i., depending on gauge, and must meet minimum elongationrequirements. In addition, it is highly desirable that the steelreinforcing wire possess a high degree of corrosion resistance, usuallyobtained by a coating step. The preponderance of steel core wire inpresent day ACSR is produced by a hot dip galvanizing process which isundesirable from a number of standpoints, not the least of which is theshortened life in corrosive atmosphere.

While it has long been recognized that it would be desirable to producea pure aluminum coated steel wire characterized by a very high tensilestrength, it has not been accomplished under prior art practices. In thefirst place, it is understood that the base metal will normally besubjected to a thermal cycling during the coating process which willgreatly reduce its original tensile strength to a level below thespecified minimum. On the one hand, it has been suggested that asatisfactory product can be obtained by alloying the aluminum coatingmetal with a second metal such as silicon. It is well known that thealuminum-silicon alloys have a lower melting point than pure aluminum,and it was therefore thought possible to run the coating process atconsiderably lower temperatures which would permit retention of thedesirable physical properties. Unfortunately, alloying the aluminum withsilicon substantially reduces the corrosion resistance of the coatedproduct.

A second solution to the above mentioned problem has been to restore thelost mechanical properties by a draw ng operation subsequent to thecoating step. This redraw- 3,526,529 Patented Sept. 1, 1970 ingoperation is then followed by a stress relieving post anneal to meet theminimum elongation requirements. As is well known in the art, subjectinga steel Wire to a severe reduction in cross-sectional area will greatlyincrease its tensile strength. However, the drawing of a coated wire isnot entirely satisfactory for various reasons. Other than the annealrequired after redrawing, the primary problem is that the soft aluminumtends to accumulate at the die entrance and may pinch off the wire.

Still another solution advanced by the prior art involves theutilization of complex salts as a flux, which, by eliminating thenecessity for furnace cleaning, permits the retention of a high degreeof the original mechanical properties of the wire. The problem is thatcomplete removal of the flux during coating is extremely difficult, andresidual flux on the wire after coating impairs the corrosion resistanceof the product.

In view of the foregoing comments, the specific objects of thisinvention may be set forth as follows:

It is a primary object of the invention to provide a process by means ofwhich a ferrous, strand-like article having predetermined physicalproperties may be coated with essentially pure aluminum whilemaintaining an acceptable level of the original tensile strength.

A further object of this invention is the provision of a process wherebya high strength alumnium coated steel wire may be produced which ischaracterized by superior corrosion resistance.

Another object of the invention is to provide a method for coating aferrous strand-like article with essentially pure aluminum whichrequires no pre-fiuxing step.

Still another object of the instant invention is the provision of amethod which may be used to produce an aluminum coated steel base wiremeeting ACSR standards and which does not involve a redrawing operation.

Still another object of the invention is the provision of a processwhich will effect the above noted objections in an economical andcommercially feasible manner.

SUMMARY OF THE INVENTION The practice of the invention contemplates thata continuous strand of cold drawn Wire having the desired physicalproperties is subjected to a series of cleaning operations, which willthoroughly clean the surface of the strand. The cleaned strand is thenintroduced into a furnace having a reducing atmosphere and operatedunder a fairly narrow and precisely controlled temperature range set outmore fully hereinafter. Without re-exposing the strand to the outsideatmosphere, it is then passed through a pot of molten coating metal. Thetemperature of the coating metal, and the time of immersion of thestrand therein are very important aspects of the instant invention. Theapparatus described in UJS. Pat. No. 3,354,864 in the name of Earle L.Knapp, may be utilized to carry out the coating step in the instantprocess. As soon as possible after emerging from the coating bath, thewire must be quickly cooled to solidify the coating and retain thedesired physical strength in the coated product.

BRIEF DESCRIPTION OF THE DRAWING The single figure schematicallyillustrates one embodiment of a wire coating line according to thisinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The starting material for thepractice of the instant invention is preferably high carbon steel whichhas been cold drawn to a tensile strength of approximately 260,000p.s.i. The acceptable range of carbon content for the starting materialappears to be rather narrowly limited, it has 3 been determined thatsteel wire having a carbon content varying from .60% to .90% is entirelysatisfactory, but it will be understood that the chemistry of the strandcan be varied outside this range with the addition of appropriatealloying elements. The pre-coated strength of the base metal strand may,of course, vary within a relatively wide range, and in securing theas-coated strength range set forth earlier, the initial strength is notparticularly critical.

A steel wire satisfactorily meeting the above noted requirements will beled from a pay-ofl reel 11 through the various operations of thisinvention.

The first stage of the invention, broadly considered, serves tothoroughly clean the surface of the wire strand. In the first step ofthe exemplary cleaning operation, carbonaceous drawing compounds areremoved from the surface of the wire. This is accomplished by passingthe wire through a Mar-Temp salt bath 12 (consisting essentially ofsodium nitrate and sodium nitrite) maintained at a high enoughtemperature to heat the wire to a temperature in the range of 700 F. to1000 F. Various conventional steps, such as a lead oxidizing bath, orsimply heating the strand in an oxidizing furnace, may be used so longas these temperature conditions are met.

Upon leaving the salt bath 12, the Wire passes under the warm sprayrinse 13 and into the acid pickle bath 14 which will remove drawingcompounds impressed in the surface of the wire during drawing. Theprecise strength and temperature of the acid solution are not absolutelycritical to this invention. A solution containing 8% HCl and maintainedat 130 F. will provide acceptable results. This acid pickle bath, inaddition to removing drawing compound particles encrusted in the surfaceof the steel wire, will also remove the surface oxide which will resultupon leaving the salt bath, and provide a bright wire surface.

Before passing into the reducing furnace, it is necessary to removepickle residue from the Wire 10. This may be accomplished by the coldwater rinse 15, the brush scrubber 16 and a hot water rinse as at 17.The hot water rinse 17 is largely self drying. In some cases, the brushscrubber 16 can be omitted, the residual acid being removed by a highpressure alkaline spray. Where the scrubber 16 is utilized, it ispreferred that the rinse solution used at 17 be very slightly alkaline,so that a very thin film of alkaline material will be left on wireentering the dryer to minimize rusting while the surface of the wire isdrying.

There are obviously many other satisfactory methods of accomplishingthese pre-cleaning steps, including the use of an aqueous alkalinesolution, sodium hydride, shot or sand blast and the like. Theseparticular steps set forth above are not critical as such to thisinvention.

The wire 10 is next led over a suitable guide sheave 18, and into theheat resistant tube 19 mounted within the furnace 20. Suitable gas, suchas dissociated ammonia, will be introduced into the tube 19 from aninlet not shown in the drawing, to provide a reducing atmosphere.

In the tube 19, the wire will be heated to a temperature whichapproaches but does not exceed 1220 F. As is known by the skilled Workerin the art, this is below the melting point of substantially purealuminum, so that at this point, wire heat is insufficient for properfinishing of the coating; that is, there is not enough latitude in thecoating metal temperature to afford a suflicient fluid fihn of moltenmetal around the wire. Proper finishing refers to the controlledapplication of molten coating metal on the wire so as to form a smooth,uniform, concentric coating of desired thickness as more fully describedin US. Pat. 3,354,864. The details of the heat balance will be explainedin more detail in connection with the discussion of the temperature ofthe bath of coating metal.

Without re-exposing the wire 10 to the atmosphere, it is passed from thetube 19 in the furnace into a bath of molten aluminum 2].. The preferredcoating apparatus has been set forth in detail in the patent referred toabove.

As stated at the outset of this application, superior corrosionresistance is attained by utilizing a bath of commercially purealuminum, and hence the pure coating metal is preferred. The termcommercially pure aluminum refers to aluminum containing onlyinsignificant amounts of impurities. It is of course, possible to alloythe aluminum bath to provide greater latitude in meeting ACSR standards.

It will be noted from the accompanying schematic drawing, that the wire10' passes directly and free of support from the reducing atmosphere intube 19 into the coating bath. This is important in insuring that wiretemperature is continuously increasing from the time it enters thereducing furnace until it leaves the coating bath, and re sults in avery simple coating line characterized by the lack of guide sheaves andthe like at this stage, thereby eliminating strip deflection, drag, andattendant difficulties.

The time-temperature relationship during immersion of the steel strandin the aluminum bath is also critical to this invention. As explainedbefore, the exit temperature of the wire from the reducing furnace willbe below 1220 R, which is also below the fluid point of commerciallypure aluminum. The Wire must therefore be heated to final finishingtemperatures (1220 to 1250") by the molten aluminum.

The time-temperature relationship required during the actual coatingstep is one which will increase the surface temperature of the wire toover 1220 F. so that finishing can be accomplished. Optimum conditionsoccur when the wire is heated to a temperature of just over 1220 F., butunder commercial conditions, this would be practically impossible.Therefore, for commercial operations, the practical wire surfacetemperature range is 1220 to 1250 F. High temperatures are possible butrather impractical in obtaining high as-coated tensile strength, due tolack of rapid enough cooling or quenching means.

By way of brief summary, the wire surface temperature is a function ofthe temperature attained prior to entering the coating bath, thetemperature of the bath, and the time of immersion therein. In otherWords, the temperature of the wire upon entering the coating bath, plusthe superheat added by the bath itself, must be suflicient to bring thesurface of the wire up to finishing temperature, wherein there is enoughheat in the wire at the time it leaves the coating bath so that there isa fluid film of metal on the surface of the wire for subsequentfinishing. These factors can be varied as desired, so long as a wiresurface temperature in the above range during coating is obtained.

The provision of some means for effectively solidifying the aluminumcoating, and cooling the wire immediately after emerging from the bath21, is essential to retain an adequate as-coated strength. To this end,the drawing illustrates a short gas finishing chamber 22 followedimmediately by a water quench 23.

The coated wire may then be taken up by a winding reel 24 for storage orfurther processing.

In order to set forth a specific embodiment of this method, thefollowing data from an actual laboratory run will be given. The startingmaterial was .101 gauge wire having the following pertinent chemistry:

The wire had been cold drawn to an initial tensile strength of 266,000p.s.i.

The cleaning steps were accomplished as generally set forth earlier inthis specification, and included immersion in a lead oxidizing bath at atemperature of 1000 F. for a period of about 4 seconds.

The wire attained a surface temperature of 1150 F. in the reducingfurnace just prior to entering the coating bath. The bath (of purealuminum) was maintained at 1260 F., and the immersion time was limitedto 1% seconds. Under these conditions, the Wire temperature was raisedslightly over 1220 F., and good coating was obtained. It should beemphasized that, as noted below, this critical wire temperature may bereached in a shorter immersion time by using a higher bath temperature.

Analysis of the coated wire showed the following physical properties: 1%yield strength, 165,700 p.s.i.; ultimate tensile strength, 187,200p.s.i; and elongation in inches, 4.5%.

In another exemplary run using generally the same starting material, aMar-Temp salt bath in place of the lead bath of the preceding example,the aluminum bath was maintained at 1300 F., and immersion time waslimited to 0.4 second. Analysis of this wire showed the followingphysical properties: 1% yield strength, 167,000 p.s.i.; ultimate tensilestrength, 190,000 p.s.i.; and elongation in 10 inches, 5%.

Briefly reviewing the thermal cycling to which the wire has beensubjected, it is first heated to a temperature in the range of 700 F. to1000 F. in the salt bath, then is cooled to a relatively lowtemperature, before being continuously heated so as to arrive at thenecessary coating temperature while in the bath of molten coating metal.It is believed that the first heating step is in the nature of apre-tempering or stress relieving treatment, and it has been determinedthat this pre-tempering operation very definitely and materially raisesthe elongation or ductility of the coated wire and helps to retain colddrawn strength during the second heating step.

Having thus described this invention in terms of an exemplaryembodiment, it should be apparent to the skilled worker in the art, thatnumerous changes and modifications are possible. Accordingly, nolimitation is intended except insofar as set forth in the followingclaims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. A continuous process for coating 2. high tensile strength ferrousstrand with commercially pure aluminum to produce a coated producthaving a tensile strength in excess of 165,000 p.s.i. comprising thesteps of:

(a) thoroughly cleaning the surface of said strand while maintaining thesurface of said strand at a temperature below 1000 F.,

(b) continually heating said cleaned strand in a reducing atmosphere toa maximum temperature approaching but not exceeding the 1220" F. meltingpoint of aluminum, said maximum temperature being attained atsubstantially the end of said heating step,

(c) immediately thereafter passing said strand through a bath ofcommercially pure molten aluminum, the speed of passage of said strandthrough said bath being related to the temperature of said bath wherebythe surface of said strand is heated to a temperature in the range from1220 F. to 1250 F., and

(d) quenching said coating.

2. The process claimed in claim 1, wherein said strand passes directlyand free of support from said heatingreducing step to said bath ofmolten aluminum.

3. The process claimed in claim 1, wherein the step of thoroughlycleaning the surface of said strand is accomplished by heating saidstrand to a temperature in the range from 700 to 1000 F., and includingthe step of cooling said strand prior to heating said strand in areducing atmosphere.

4. The process claimed in claim 1, wherein the step of thoroughlycleaning the surface of said strand is accomplished 'by first heatingsaid strand to a temperature in the range from 700 F to 1000 F. to burncarbonaceous material from the surface thereof, then subjecting saidstrand to an acid pickle to remove particles of drawing compoundencrusted in the strand surface, and thereafter rinsing said strand toremove residual pickle acid and smudge.

References Cited UNITED STATES PATENTS 2,926,103 2/ 1960 Brick 117-512,197,622 4/1940 Sendzimir. 2,294,750 9/ 1942 Harris. 2,372,599 3/ 1945Nachtman. 2,592,282 4/1952 Hodil. 3,013,899 12/1961 Price et al.3,057,050 10/1962 Hodge et al. 3,082,119 3/ 1963 Harris. 3,227,5771/1966 Baessler et al. 3,259,148 7/ 1966 Krengel et al. 3,322,560 5/1967Monaco.

ALFRED L. LEAVITT, Primary Examiner J. R. BATTEN, 111., AssistantExaminer US. Cl. X.R.

